Background Information

Many materials are hydrophobic (water-fearing) in nature. Due to their non-polar chemical structure, hydrophobic particles want to minimize contact with polar (water) molecules and, as a result, tend to aggregate on the surface of the water and resist going into suspension. This presents a challenge to scientists and engineers who would like to be able to work with hydrophobic particles suspended in aqueous solution.

Fortunately, there is a simple way to overcome the hydrophobic effect. It is called a surfactant, a detergent, or simply “soap.” Surfactant is a magical molecule that has both hydrophobic and hydrophilic properties, which coats the particles and helps them mix into water. The same mechanism applies when we use soap to wash greasy dishes or stained clothes.

Selection of the surfactant depends purely on your process and product requirements. Dishwashing liquid works great, so does Simple Green. For scientists working on biological applications we recommend the use of Tween surfactants. Tween is the commercial name for Polysorbate non-ionic surfactants, which are stable, nontoxic, and often used in pharmacological, cosmetic, and food applications. Non-ionic detergents are considered to be “mild” detergents because they are less likely than ionic detergents to denature proteins. By not separating protein-protein bonds, non-ionic detergents allow the protein to retain its native structure and functionality.

Tween 20 and Tween 80 are frequently used. Both surfactants are yellowish, water-soluble viscous liquids. Primary difference between the two is viscosity. Tween 20 has lower viscosity and is easier to work with.

Suspension Process

There are many ways to suspend the particles (e.g. put a few drops of dish detergent into water and shake with the particles).

The process below is specific for using the minimum amount of Tween for biologically sensitive applications.

Safety:

Gloves and eye protection are to be worn at all times during solution preparation and use.

Care should be taken when handling hot objects/liquids and immersion blender.

Centrifuge should be properly balanced and allowed to come to a full stop before opening.

Recommendations:

We recommend using distilled water to minimize impurities.

We recommend boiling the water to sterilize and to make it easier to disperse a small amount of surfactant uniformly. This also increases shelf-life of prepared solutions and suspensions.

We use an immersion blender to disperse the surfactant in water quickly and effectively.

Process:

Preparing Tween Solution:

Fill a heatproof container with distilled water.

Ensure the water level is high enough to cover the immersion blender.

Heat water to boiling and leave boiling for 5 minutes.

Weigh out 0.1g of Tween per 100ml of water used (creating 0.1% solution).

Slowly add Tween to boiled water while mixing with immersion mixer (~30 seconds).

Some bubbles will form during mixing.

Bubbles will dissipate on cooling and solution will appear clear.

Suspending particles in Tween solution.

Place the desired amount of particles into a container.

Dispense prepared Tween solution on top of particles.

We recommend at least five times greater volume of solution to the volume of particles.

Cover tightly and place containers into a centrifuge.

Centrifuge on highest setting for at least 5 minutes.

If some particles are still floating on the surface of water, more centrifuging may be necessary.

A small quantity of particles may accumulate on the top surface and not enter solution despite additional centrifugation. Typically, these particles will go into suspension over time (hours).

Other Considerations

A greater length of centrifuging or larger volume of Tween solution may be necessary to suspend certain materials and particle sizes.

As a 0.1% Tween solution is sufficient for most applications, concentration levels could be raised to support particles that are more resistant to entering solution.

Once the particles are suspended, solution can may be diluted further to increase the volume.

Particles can be recycled and reused as necessary. The suspension might need to be repeated.

If no centrifuge is available, it is possible to shake the container by hand (up and down, upside down) to achieve the same result.

Here is an example of Cospheric fluorescent beads 150 to 180micron in diameter being dispersed in a pilot bioreactor.

About Cospheric

Our extensive product line consists of more than two thousand unique spherical microparticle and nanoparticle products, all developed based on customer demand. We work with each individual customer to find a creative solution for their unique needs ­– tight particle size ranges, wide selection of colors, densities, properties and formulations. We are the sole global supplier for the majority of our products. We developed a disruptive technology which is redefining the microsphere market and creating a new category of precision spherical particles. Our research department is always excited to tackle new challenging projects. Explore at www.Cospheric.com.

Other Information

The information contained in this document is correct to the best of our knowledge at the date of publication. It should not be viewed as all inclusive, but as a guide only. It does not represent any guarantee of the properties of the product. Cospheric LLC shall not be held liable for any damage resulting from handling of or from contact with the above product. For these reasons, it is important that product users carry out their own tests to satisfy themselves as to the suitability of the safety precautions for their own intended applications.

Particle Image Velocimetry – Particle Density and Neutral Buoyancy

Density of particle vs. density of fluid:

Silver Metal Coated Hollow Glass Microspheres

Many flow visualizations are done in water but there are a variety of other liquids and even water based solutions that will have differing densities whose velocity fields are being mapped or will be in the future. Therefore, having seed particles to match these differing densities is important to provide neutral buoyancy, one of the most important factors regarding flow conformity.

With polyethylene spheres contributing a range of densities that provide neutrally buoyant tracers for most water fluid flows thought needs to be directed towards furnishing tracers to match other densities. Gasoline and similar carbon based fluids are an example of areas of fluid flow mechanics where lower density tracer particles are necessary. Silver Coated Hollow Glass Microspheres have the density range to potentially work for many carbon chain liquids, with current densities ranging from .15 g/cc to .75 g/cc and possibly higher. This leaves some room for improvement in the densities between .75 g/cc and .96 g/cc.

There are many options available for neutrally buoyant microspheres in water. While, other fluids may have more difficulty finding an ideal density tracer, there are options available and scientists are working to provide more unique and targeted tracer particles for use with low density fluids.

Particle Image Velocimetry – Particle Size and Distribution

Particle size:

Particle size is connected to many of the other parameters of seed particles in a PIV system. With size affecting visibility, flow conformity, and being integral in relation to pixel size. A rough number for ideal particle size is 1-100um though sizes in the nm and mm’s have been used for certain PIV applications. With smaller sizes being necessary for micro-PIV methods and larger sizes being a requirement for large scale flow visualization. The importance of size is related to how truly the tracer will follow the flow with particle diameter having the largest effect on stokes number, which is a representation of flow tracer fidelity. Though when particle size becomes too small it can be difficult to confirm that the tracer is not being affected by minor currents or other factors within the fluid. Also as size decreases visualizing the spheres can become quite challenging. However, the stokes number can provide a decent representation of how well particles follow the flow. Though, the stokes number is an approximation based on assumptions and therefore can only provide a useful representation rather than a confirmation of tracer fidelity.

Particle size distribution:

Fluorescent Red Polyethylene Tracers

A parameter that should be considered in conjunction with particle size is distribution. As particles in the sizes used for PIV are so small that no meaningful quantity of tracers can be produced in a specific size and rather size ranges need to be considered. With tighter size distributions, there will be less error attributable to differences in visibility of particles and a better approximation of how well each particle being used will conform to the flow. For example fluorescent red polyethylene has multiple size ranges available (10-22um, 10-45um, 10-90um, and 10-150um). With tighter size distributions being more difficult to obtain and as such being more expensive. Raising the question of what the trade off between price and size distribution is. Wide distributions can be used within PIV, however they may necessitate further image processing and may reduce accuracy of measurements. Therefore, there is no perfect size distribution choice. Though, with the understanding of what is available the choice of a correct size and size distribution can be determined.

Particle Image Velocimetry – Intro to Tracer Particle Parameters

PIV is a vast field with varying techniques and differing areas of research. Techniques vary from 2D PIV, only viewing velocity in a plane of the fluid system, to high speed TOMO PIV which views a 3D area of fluid and can be time resolved allowing for acceleration data to also be obtained. Another difference is that the size of liquid PIV set-ups can range from micron sized micro channels to multi thousand-gallon tanks. While the area being imaged may not vary as much as the

Barium Sulfate Tracer for X-ray imaging

systems themselves, it can still differ from units of micro meters to potentially meters. With viewing windows growing as new advancements in science and technology progress, the need for seed particles to match them will grow. One example of this is the rise of helium filled soap bubble seeders that provide an easily visualized 300um bubble for air systems allowing for large areas to be seeded and visualized. Or barium sulfate polyethylene microspheres which are useful due to being a radio contrast agent allowing for visualization via x-ray imaging.

Therefore, a one solution fits all approach is not feasible when it comes to seed particle selection. As each experiment will have differing size, density, light intensity/visibility, particle material, and seeding concentration needs based on desired results.

Particle Image Velocimetry and Tracer Particle Visibility

Particle Image Velocimetry (PIV) expresses a vast field with varying techniques and data acquisition methods. However, the main goal is providing an optical method of flow visualization. The exact information obtained depends on which method is used, with new algorithms and approaches being discovered constantly.

There are generally two ways data is obtained PIV and Particle Tracking Velocimetry (PTV) which can then be broken down into many other methods based on how exactly the data was obtained and the processing done to said data. PIV measures the velocity field of a fluid based on a Eulerian method where stated locations are observed over time to determine the flow. While PTV tracks the movement of singular particles over time, a Lagrangian approach. This provides a plot of the particles movement and by relation information about the fluid flow. They each use the same tracer particles however they look at them in different senses. If logs in a river are representations of our seed particles then PIV looks at the river and sees the logs moving through it determining how the river flows based on this information. While PTV watches the movement of individual logs to obtain similar information. Which leads to the assumption that tracking particles must be easily visualized.

Visibility being an important aspect of tracer particles is a given but how those particles are visible is where differences can come about. Tracers can be visible if they block light from reaching the visualization mechanism (eye, camera, etc.) essentially being visible as a shadow. This method is known as backlit shadowgraphy where the flow is placed between an illumination source and a camera allowing for the absence of light (shadow) caused by tracer particles to be tracked.

Reflective Silver Coated Hollow Glass

Another approach to assuring particle visibility is using highly reflective spheres that will reflect in the direction of your camera set-up allowing them to appear as dots of high intensity light, of the illumination source used. Lasers are most commonly used as the illumination source for this form of particle visibility. As lasers have high power, high collimation, and a relatively tight emission bandwidth. Recently LED’s are also being used as the illumination source for reflectivity visualization methods as well as backlit shadowgraphy. LED’s may not currently have the power or collimation abilities of lasers but are consistently growing in power. LED’s also have a very limited emission spectrum as well as their ease of use and low cost compared to lasers.

Fluorescent Tracer Particles

Finally, some tracer particles can emit their own light which allows them to be an easily distinguishable wavelength from the illumination source which can often flood the visualization area. One of the most common examples of this would be fluorescent spheres. Which when excited by the illumination source will emit a different wavelength of light. This allows the wavelength of light used as your illumination source to be filtered out providing an image with just the light from tracers. Phosphorescent spheres fall into a category similar to fluorescent particles as phosphorescence emits light similarly to fluorescence. However, phosphorescence emits over longer periods of time. Another significant difference of phosphorescent materials is their unique temperature variance which allows for them to be used as a form of temperature sensor.

With both PIV and PTV having their strengths and weaknesses there is no clear superior method. However, with advances in technology PTV is becoming more feasible and thus may overtake PIV methods due to its ability to provide greater data varieties. Visibility options also have their unique aspects that ensure their necessity in specific cases. Shadowgraphy is gaining traction in areas due to its reduced cost requirement and ease of use. While, fluorescent tracers remain as an ideal option for applications where shadowgraphy can not quite meet the necessary criteria.